Rename quack to quak

German for 'quack', matching the Ente (German for 'duck') naming. All
references updated: package name, CLI binary, X-Client-Package header,
test descriptions, temp dir prefixes, README, Makefile docker tag.

test/model/decrypt.test.ts

@@ -3,7 +3,7 @@
  *
  * These two functions turn the raw encrypted JSON blobs the Ente server
  * returns into the decrypted Collection and EnteFile objects that the
- * rest of quack works with.
+ * rest of quak works with.
  *
  * ## Collection decryption
  *